The present invention relates to propeller devices and in particular, relates to variable pitch propeller devices for airplanes.
During flight, the pitch of propellers of variable pitch propeller airplanes can be advantageously adjusted to adapt to various flight conditions. For example, the propeller is normally set at low pitch on takeoff, while climbing, and upon landing; high pitch when cruising and feathered if engine failure should occur.
Such variable pitch propeller devices are described in U.S. Pat. Nos. 1,936,677; 2,425,261; 2,553,128; 2,554,611; 2,717,652; 3,095,932; 3,024,848; 3,056,457; and 3,338,313. Other patents relevant to the variable pitch propeller device described herein are U.S. Pat. Nos. 1,125,717; 1,777,254; and 3,792,937; and British Pat. No. 569,867.
One prior art variable pitch propeller device comprises generally a hydraulically controlled adjusting mechanism positioned in the propeller hub. The mechanism is connected to the aircraft's hydraulic system through channels formed in the propeller shaft. Unfortunately, it is difficult and expensive to bore channels in the propeller shaft. Further, in some cases, it is difficult to correctly position the channels to accurately balance the propeller shaft. Another problem with boring channels in the propeller shaft is that with small airplanes, i.e., airplanes having engines with less than about 150 horsepower, the propeller shaft generally has inadequate structural integrity to be bored without risking failure during flight.
Another prior variable pitch device is disclosed in the aforementioned U.S. Pat. No. 2,554,611 which is issued to Biermann. Biermann discloses a hydraulically controlled mechanism which is secured to the engine crankcase. The mechanism is connected to a thrust pin which runs parallel to the propeller shaft and is connected to the base portion of each propeller blade. The hydraulically controlled mechanism actuates the thrust pin to vary the pitch of the propeller. Unfortunately, the pitch of the propeller can only be varied within a narrow range due to the juxtaposed position of the thrust pin to the propeller shaft. Further, because of the substantial modifications to the engine body and cowling required for installation of the mechanism, it is economically prohibitive to modify a fixed propeller airplane with retrofit equipment.
In the aforementioned U.S. Pat. No. 2,425,261, which is issued to Murphy et al., there is described in column 1 prior art hydraulically actuated propeller blade pitch changing mechanisms. It is noted by Murphy et al that it would be desirable to have the hydraulic mechanism located between the propeller and engine. Murphy et al. state that:
"In a class of this design, the hydraulic unit consists essentially of a stationary cylinder and non-rotating piston. This construction is obviously objectionable because of the fact that the total force exerted by the piston must be transmitted to the blade turning mechanism, located on the propeller hub, through the medium of a thrust bearing running at the same high speed as the propeller. To those familiar with the art, a bearing which would meet such severe operating conditions is too heavy and difficult to lubricate and also requires too much space to be adaptable to practical use for the purpose concerned."
In view of these problems, Murphy et al developed a device using a hydraulic cylinder concentrically mounted with the propeller shaft.
In spite of these problems with a stationary hydraulic unit mounted between the engine and the propeller, such a device retains certain attractions, because it has the potential for use on a large variety of airplanes.
Therefore, there is a need for a variable pitch propeller device which can vary the pitch of the propeller over a wide range, which can be used without modification of an airplane and its engine, permits the hydraulic unit to be mounted between the propeller and the engine, and can readily be used for a wide variety of airplanes, and in particular airplanes having engines of about 150 horsepower and less.